You can just use a bag with the coupler on the inside and no tip
You can buy plastic, and cut it in to your own shape balloon, or there are a ton of birthday candles that you can use shaped like balloons!
Try getting a ballon cookie cutter for a stensil otherwise make a stensil out of cardboard
Here is a page on birthday cakes including a balloon cake. Www.birthdaycakeideas.info/